Access may be restricted in many ways. Generally some type of mechanical or electronic lock may be used to restrict access to only those having the appropriate mechanical key, lock combination, personnel identification number (PIN), and the like. Mechanical locks, accessible by a key and/or rotating wheel combination, have been used for years and predate electronic locking means. Computers, cash dispensing kiosks, electronic access panels, etc., may use unique combinations of numbers and/or letters that must be entered by an individual wanting to access what the unique combination is protecting.
However, all of these access restriction devices suffer from the ability of unauthorized persons to be able to defeat the keys, combinations and/or codes that are supposed to make these restricted access devices secure. For example, a keypad access system may be compromised by dusting the keyboard with a fine invisible powder only visible with a special light or chemical. Then once the numbers are known, it is a simple matter to try all potential combinations thereof. Also the keypad may be observed with a camera and/or binoculars and the numbers recorded that are pushed when an authorized user is inputting the correct codes. Putting the keypad in a less obvious place may make it difficult for the user to see the keypad and/or compromise the user's situation awareness in an area prone to muggers.
Mechanical combination locks use noisy tumbler mechanisms that allow an unauthorized person to listen to the tumbler mechanism and thereby determine the lock combination. Key based access control mechanisms suffer from the ability of an unauthorized person to “pick” the lock or make use of a “bump” key to defeat the lock. Hand and eye scanners may be used as security devices but suffer from being expensive and mechanically complicated. May be compromised by removal of the user's eyeball or severance of a hand. An authorized user may leave physical evidence of a hand print.